Refrigeration and air conditioning systems have long employed helical screw rotary compressors as an element within a closed loop refrigeration circuit, with the compressor, condenser and evaporator connected in that order in series within the closed loop and with a thermal expansion valve or similar expansion means intermediate of the condenser and evaporator and thereby defining system high and low side pressure to opposite sides of the expansion means. Further, such helical screw compressors are often characterized by an unloader slide valve which is shiftable longitudinally to the screw compressor casing and forming a part of the envelope for the intermeshed helical screw rotors, wherein the compression process takes place. Such slide valves are fixedly coupled to a piston which is sealably carried within an unloader slide valve linear drive cylinder aligned with the slide valve and extending from the compressor casing. The slide valve itself is shiftable between extreme full load and unload positions. In the unload position, a large portion of the refrigerant gas entering the compressor at the suction port is permitted to return to the suction side of the compressor to the extent of linear displacement of the slide valve from a fixed stop defining a full load position. When the slide valve shifts towards that stop, by-pass or return of the gas is restricted and the refrigerant gas entering the suction port must be compressed by the compressor which discharges at high pressure at a discharge port. The high pressure compressed gas is directed to the high side of the machine for condensation within the condenser and ultimate feed as a liquid through the thermal expansion valve or similar expansion means to the evaporator. Here vaporization of the refrigerant occurs, prior to return as a low pressure vapor to the suction port of the compressor.
Further, conventionally, oil is fed to the bearings of the compressor, and is preferably injected directly into the compression process through one or more injection ports within the compressor casing where it mixes with the refrigerant. Downstream of the compressor and upstream of the condenser, an oil separator is conventionally provided within the closed loop. Oil is separated from the refrigerant, which refrigerant then circulates in the closed loop. The oil is returned to the compressor with a portion thereof injected directly into the working chamber as defined by the intermeshed helical screw rotors. The linear drive cylinder is preferably a hydraulic cylinder, and the piston which is sealably and slidably mounted within the cylinder defines closed chambers on opposite sides. An inboard chamber is proximate to the compressor itself, and an outboard chamber is remote from the compressor. Typically, a coil spring is interposed in the compressor slide valve assembly and acts directly on either the slide valve or the slide valve drive cylinder piston to bias the slide valve into full compressor unload position providing maximum by-pass or return of the suction gas entering the compressor working chamber.
In order to effect loading of the compressor, depending upon system load conditions, the separated oil, which is at discharge pressure (or further pressurized by an oil pump), is directed to the outboard chamber to drive the slide valve in a direction tending to close off the by-pass opening or gap between the slide valve and the fixed stop, i.e., towards full load position. While this system operates fairly satisfactorily in practice, it is complicated and is subject to possible problems should the spring break or hang up. Additionally, in order to shift the slide valve in opposition to the spring bias, some work must be overcome, therefore providing, at least to some extent a power loss.
It is, therefore, a primary object of the present invention to provide an improved unloading control system for a helical screw operated, closed loop refrigeration or air conditioning system which is simple in operation, which is automatically effected during compressor shut down and in which, the need for a spring for biasing the slide valve to unload position is eliminated.